Santa Tecla, 2-9 October 2005Marita Mosconi,FZK1 Re/Os cosmochronometer: measurements of relevant Os cross sections Marita Mosconi 1, Alberto Mengoni 2,

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Presentation transcript:

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK1 Re/Os cosmochronometer: measurements of relevant Os cross sections Marita Mosconi 1, Alberto Mengoni 2, Michael Heil 1, Ralf Plag 1, Franz Käppeler 1, Kaori Fuji 5, Gaelle Aerts 3, Rita Terlizzi 4 and the n_TOF collaboration 186 Os, 187 Os and 188 Os (n,) cross sections at the n_TOF facility operating at CERN 1 Forschungszentrum Karlsruhe GmbH (FZK), Institut für Kernphysik, Germany 2 CERN, Geneva, Switzerland; IAEA, Vienna, Austria 3 CEA/Saclay, Gif-sur-Yvette, France 4 INFN Bari, Italy 5 INFN Trieste,Italy Inelastic scattering cross section of 187 Os at 3.7 MV FZK Van de Graaf

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK2 s-process local approximation:  A N A =  A-1 N A-1 At stellar temperatures, 187 Os low-lying excited states are populated: theoretical calculations are needed to evaluate the stellar capture rate The Re/Os cosmochronometer

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK3 187 Os stellar rate 187 Os 3/2 - 5/2 - 7/2 - 1/ keV 74.3 keV 75.0 keV keV Thermal population at 30 keV: P(gs) = 33% P(1st) = 47% P(all others) = 20% Theoretical calculations can use as input experimental results: the measurement of the inelastic scattering cross section for the first excited level allows to define the competition by the scattering channels under stellar conditions

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK4 Clock determination Nuclear quantities temperature dependence of t 1/2 ( 187 Re) (Bosch et al., Phys.Rev.Lett 77 (1996)) 186 Os, 187 Os(n,) cross sections from 0.1 keV to 500 keV inelastic scattering cross section on 187 Osevaluation of stellar cross sections including the effect of excited states (inelastic scattering cross section on 187 Os ) Astrophysics astration galactic history of the 187 Re enrichment

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK5 Measurements of the (n) cross section at n_TOF

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK6 Experimental setup at n_TOF (2003) Neutron flux monitor: Silicon detectors viewing a thin 6 LiF foil Pulse height weighting technique Correction of the -response by weighting function to make the detector efficiency proportional to -ray energy -ray detection: C 6 D 6 scintillators Spallation neutron source: 20GeV protons on a lead target providing a wide range of neutron energies C6D6C6D6 C6D6C6D6 Neutron beam Sample changer

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK7 Samples measured and yield 186 Os (2 g, 79 %) 187 Os (2 g, 70 %) 188 Os (2 g, 95 %) Al can  environmental background nat Pb (2 g)  in-beam gamma background nat C (0.5 g) neutron scattering background 197 Au (1.2g) flux normalization (using Ratynski and Macklin cross section measurements

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK8 Maxwellian averaged cross sections at 30 keV n_TOF (mb) Winters (mb) Browne (mb) 186 Os409±20418±16438± Os931±47874±28919± Os298±15401±15395±24 Preliminary results

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK9 Scattering experiment at FZK Van de Graaff

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK10 Time-of-Flight technique neutron beam with a narrow energy distribution (FWHM≤10 keV) Experimental method identification of the inelastic scattering component by energy separation measure the cross section as closer as possible to the energies relevant for s-process FWHM≤10E p -E th ≤0.3 keV θ=0° θ=30° θ=45° θ=60° θ=15° θ=25° 1. narrow neutron distributions for E n ≥ 60 keV 2. Gaussian neutron energy distribution centered at 30 keV use of 7 Li(p,n) 7 Be reaction:

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK11 Experimental setup and DAQ PMT Neutron monitor 6 Li-glass Samples measured 187 Os 188 Os pure elastic component Empty canning background improved neutron detection efficiency by use of KG2 (NE912) background subtraction by pulse shape analysis Main drawback of 7 Li(p,n) 7 Be at threshold: small yield Beam unstable due to proton source instabilities: computer monitoring of the neutron energy distribution

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK12 Data Two week of data taking with proton current 3-6 μA Runs with similar FWHM extracted Parameter of the scattering component extracted Fit of the 187 Os scattering distribution thought as made by two separated components Data analysis

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK13 Summary 186,7,8 Os(n) cross sections: background subtraction concluded resonances analysis in progress isotopic correction to be improved using the results of the previous item 187 Os(n,n’) cross sections: data taken for neutron distribution centered at 30 keV and with FWHM from 7 to 10 keV analysis in progress

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK14 The End

Santa Tecla, 2-9 October 2005Marita Mosconi,FZK15 Derivation of the cross sections in the unresolved region 1. -response corrected by weighting functions 2. background subtraction 3.neutron flux determined by Au count rate (using Ratynski and Macklin cross section measurements) 4. isotopic correction 188 Os with filters Estimated level of background neutron scattering background evaluated using the C measurement ( environmental background corrected ) rescaled for the ratio of the neutron scattering cross sections of carbon and of the sample analyzed shape of the -background determined using Pb measurement after subtraction of it own neutron scattering background and environmental background level of the -background determined by GEANT3.21 simulations Au 4.9 eV resonance SAMMY fit Background Normalisation 0.992